1. Field of the Invention
This invention relates to a printing apparatus and a method for conveying a printing medium, and in particular, to a printing apparatus that has a printing medium reversing unit for reversing a printing medium to perform two-sided printing on the printing medium, and a method for conveying such a printing medium.
2. Description of the Related Art
A printing apparatus for two-sided printing on a printing medium is generally provided with a printing medium reversing unit for reversing the printing medium.
FIGS. 1 and 2 are schematic diagrams illustrating a schematic configuration of a conventional printing apparatus, and with use of the diagrams, reversing operation of a printing medium P is briefly described.
The printing apparatus has a printing medium stack 1 for stacking the printing medium P thereon, and a feeding roller 2 for feeding out the printing medium P from the printing medium stack 1. The printing medium P fed out from the printing medium stack 1 then reaches a first conveying roller 10 via a PE sensor 20, where the printing medium P is continued to be conveyed. (Note that although the term “roller” described hereinafter refers to a configuration of a “roller pair” provided with a roller driven by a drive source and a driven roller for nipping and conveying the printing medium P in corporation with the roller, for convenience, this configuration is simply referred to as a “roller” to provide the description below. The conveyed printing medium P is then conveyed to a printing area opposing a printing head 4, where printing is performed on the printing medium P, which is then discharged to a discharge tray by a discharge roller 12.
The two-sided printing will now be described with additional reference to FIG. 3. The printing medium reversing unit R includes a loop path, on which a first reversing roller 21 and second reversing roller 22 are provided. For the exemplary device configuration illustrated in FIGS. 1 and 2, the printing medium reversing unit R is located upstream of the printing area with respect to a printing medium conveying direction for printing.
After the printing has been performed on a first side of the printing medium P, the rotation of a motor is reversed to reverse rotation of the first and second conveying rollers 10 and 11 (step S001). When reverse rotation and conveyed to a branch A denoted by a symbol A in FIG. 1, the printing medium P is conveyed from the branch A to the printing medium reversing unit configured as the loop path. FIG. 2 indicates that the fore end of the printing medium P has again reached around the branch A via the printing medium reversing unit R. By passing through the printing medium reversing unit R, printing can be performed on a second side of the printing medium P, which is the back side of the first side on which the printing has been completed. For the inversed printing medium P, the PE sensor 20 detects whether or not the printing medium P has reached the position of the PE sensor 20 (step S003). After the PE sensor 20 has detected that the printing medium P has reached there, it is further determined whether or not the printing medium P has reached a normal rotation position of the first conveying roller 10 (step S004), and if it is identified that the printing medium P has reached the position, the first and second conveying rollers 10 and 11 are rotated in normal direction (step S005) and successfully terminated. Then, printing is performed on the second surface of the printing medium P conveyed to the printing area via the first conveying roller 10, and the printing medium P is discharged to the discharge tray to complete the printing process.
On the other hand, when the drive source (or driving motor, not shown) providing driving force to the first and second reversing rollers 21 and 22 is overloaded and a servo error occurs (step S002) while the printing medium P is passing through the printing medium reversing unit R, the rotation of the driving motor is stopped to suspend the reversing rollers 21 and 22 (step S006). Then, it is determined that the servo error is caused by a paper jam, and a paper jam error is notified (step S007), resulting in error termination.
Japanese Patent Laid-Open No. H10-245140(1998) discloses a technique that detects a paper jam error when target and actual positions of the fore end of a printing medium are misaligned at a reversing unit for reversing the printing medium. In this technique disclosed in Japanese Patent Laid-Open No. H10-245140(1998), if a higher sliding load acts on reversing rollers to cause an overload on a motor for rotating the reversing rollers, it is then determined that there exists a paper jam error, and the motor is stopped.
In a printing apparatus described above having such a printing medium reversing unit, if the size of the printing medium is relatively larger than that of the printing medium reversing unit, the paper jam error might occur caused by the fore end and back end of the printing medium contacting each other. For saving space and downsizing the apparatus, for example, the above reversing unit may also be designed small. Such downsizing the printing medium reversing unit makes shorter a path for conveying a printing medium in the printing medium reversing unit. As a result, the fore end of the printing medium P having passed the loop path can jostle at the exit of the path (i.e., branch A) with an area containing its back end (the back end surface) before passing the path. Since the printing medium P is still continued to be conveyed, the fore end contacting on the back end surface is sometimes almost brought in the direction of conveying the back end surface of the printing medium P by friction and cannot exit from the loop path of the printing medium reversing unit. This situation causes abnormal rotation of the first and second reversing rollers 21 and 22 to overload the driving motor.
The technique disclosed in Japanese Patent Laid-Open No. H10-245140(1998) cannot discriminate errors caused by so-called the paper jams from those by the jostling of the fore end and back end area (back end surface) of the printing medium and determines the both cases as paper jam errors. Therefore, the printing operation must be terminated to perform paper jam fixing operations in that case, lowering the throughput and usability of the printing apparatus.
On the other hand, Japanese Patent Laid-Open No. 2003-280294, aiming to prevent paper jams caused by the jostling of the fore end and back end of the same long two-sided paper conveyed on a path, discloses a mechanism for separating driven rollers of rollers from each other on an reversing path while the paper being reversed, thereby preventing the jostling of the fore end and back end of the paper.
Applying the technique disclosed in Japanese Patent Laid-Open No. 2003-280294 to a printing apparatus enables continuing printing operations because, even when the fore end and back end of a printing medium jostle with each other, there is no load fluctuation on a drive motor and no servo error is detected. Therefore, the throughput of the printing operations does not decrease; however, additional mechanism is needed for separating driven rollers of rollers from each other at a reversing unit, complicating the entire mechanism of the printing apparatus. As a result, its original purpose of making the printing apparatus smaller and thinner cannot be accomplished.